Lance Armstrong is a seven times winner of the Tour de France AFP/Getty Images

US anti-doping authorities will not reveal the names of more than 10 cyclists and other witnesses who have given evidence against Lance Armstrong for fear they could face "retaliation" or "intimidation" ahead of a possible hearing later this year.

Armstrong and five other men once involved with the United States Postal Service team have been accused of a range of doping offences by the US Anti-doping Authority (USADA), including an act of conspiracy to cover up their actions over a 13-year period. According to the USADA charge sheet this involved the "use of fear, intimidation and coercion to enforce a code of silence by team members and employees to prevent detection."

Armstrong and the other accused have until 22 June to reply to the charges. Armstrong has vehemently denied the allegations, describing USADA as operating maliciously and conducting a "witch-hunt" and "vendetta" against him.

A review board will consider USADA's case along with any response from the accused and decide whether it should go before a hearing, probably in the autumn. The witnesses will remain anonymous at the review stage, say USADA, because it "is important to shield them from the retaliation and intimidation that co-operating witnesses have faced in other matters relating to the USPS conspiracy."

The witnesses are likely to include Floyd Landis and Tyler Hamilton, who have both served bans for doping offences and have in the past accused Armstrong of taking illegal substances.

It is the latest allegation to be made against one of cycling's greats, a seven-time winner of the Tour de France and one of the most high-profile athletes in the US. If the charges are upheld he faces a lifetime ban from any involvement in sport. Armstrong, now a triathlete, was yesterday forced to pull out of an event in France because of the USADA action.

A 15-page email sent by USADA to Armstrong and the other accused outlining the charges was published by the Wall Street Journal. It contains allegations of long-term systematic doping. USADA describe it as "persuasive pattern of doping." After the email was leaked, USADA's chief executive, Travis Tygart, confirmed it was the first step in a "multi-legal process". He said: "USADA only initiates matters supported by the evidence. We do not choose whether or not we do our job based on outside pressures, intimidation or for any reason other than the evidence."

Armstrong is accused of six doping violations: use and/or attempted use of prohibited substances; possession of prohibited substances; trafficking; administration or attempted administration to others of prohibited substances; assisting, encouraging, aiding, abetting, covering up and other complicity involving one or more anti-doping rule violations; and aggravating circumstances.

The email says "numerous riders, team personnel and others will testify [that they observed] Armstrong dope or through Armstrong's admissions of doping to them that Lance Armstrong used EPO, blood transfusions, testosterone and cortisone during the period from before 1998 through 2005."

USADA also claim a urine sample Armstrong gave in 2001 was "indicative of EPO use" but the result had been covered up and that blood collections taken in 2009 and 2010 suggest "blood manipulation including EPO use".

As part of the attempt to cover up the violations Armstrong and the others are alleged to have given "false testimony under oath and in legal proceedings."

Armstrong, who retired in February 2011, released a statement. It said: "These are the same charges and the same witnesses that the [US] Justice Department chose not to pursue after a two-year investigation. These charges are baseless, motivated by spite and advanced through testimony bought and paid for by promises of anonymity and immunity. I have never doped."

The other accused are Johan Bruyneel, then team director of the USPS, Pedro Celaya, a Spaniard now RadioShack's team doctor, Luis del Moral and Michele Ferrari, respectively Spanish and Italian doctors who used to work for USPS, and Pepe Marti, formerly USPS's team trainer. All deny the charges.